Generally speaking, light emitting diodes (LEDs) are finding their way into an ever increasing variety of light fixtures for an ever increasing number of applications. The popularity of LEDs has been driven by a number of factors, such as: a heightened awareness of the ecosystem spurred by the so called “climate change” debate; increased efficiency which can realize a rapid financial payback typically measured in months; exceptionally long bulb life compared to other lighting options; visually pleasing light quality; and an ever decreasing price in dollars per lumen of output. This list is not exhaustive and virtually every application for LED lighting will find advantages specific to the application.
LEDs offer significant advantages over every other known type of bulb. For example, LEDs produce at least four times the light produced by an incandescent bulb of the same wattage. However, if one measures harnessed light, the light actually striking an illuminated object, the LED typically delivers close to eight, sometimes as much as ten, times the light. This is due to the fact that the light from an LED is usually delivered conically rather than spherically, eliminating the need to reflect light headed toward the back of the fixture, an inherently inefficient process.
LEDs are also environmentally attractive compared to fluorescent lighting, or other gas discharge-types of lighting. While efficiencies of LEDs and gas discharge bulbs are similar, when measured in lumens per watt, harnessed light from LEDs is typically close to twice that of gas discharge-type bulbs. Further, LEDs have not reached their theoretical limit of efficiency, and indeed newer models continue to improve output, e.g., lumens per watt. In contrast, gas discharge bulbs are more fully evolved. In addition, gas discharge bulbs, including fluorescent tubes, use small amounts of mercury. There is growing concern over filling landfills with mercury. Admittedly, chemicals are used in the making of LEDs that may not be any less dangerous than mercury but: 1) LEDs are hermetically encapsulated so the chemicals will not find their way into the environment, unlike the glass envelope of gas discharge devices, which are easily broken during disposal; and 2) the bulb life of LEDs is significantly longer so radically fewer LED devices are finding their way into landfills in the first place.
In particular, LEDs are proving beneficial for all types of high-power lighting, such as street lights, parking lot lights, movie and television production lighting, theatre lighting, indoor high-bay lighting, projector lighting, and the like. There are many reasons for the switch to LEDs but in many of these applications fixtures are inaccessible and bulb life is of primary concern.
In the case of entertainment lighting, LEDs have proven themselves to render skin tones with exceptional accuracy, making the LED particularly useful for motion picture and television production. In addition, unlike gas discharge lamps, LEDs can easily be dimmed over their entire range of operation and, unlike incandescent bulbs, LEDs have only a small color temperature shift over the dimming range. Further, the absence of any significant amount of infrared radiation from LEDs means there is virtually no heat felt by the subject of the illumination.
These markets have created a demand for higher and higher wattage LEDs. For many years, the limiting factor for the power of an LED fixture was the ability to dissipate heat from the LEDs into the environment. Other types of lighting radiate most of the heat produced by the bulb as infrared energy. In contrast, LEDs produce virtually no infrared. While in many ways this is an advantage, i.e. people under the light do not get hot, in some ways it is a disadvantage: all of the heat from LEDs must be conducted away. Fixture designers must strive to keep the LED die at less than 85° C., excessive temperature results in significant reduction in the life of the LED and, in the case of white LEDs, premature failure of the phosphors. Many LEDs exhibit a permanent shift in color temperature toward warmer hues when the LED is operated above its rated temperature.
For smaller LEDs the limiting factor is the amount of heat that can be conducted away in the electrical leads. In higher power devices, a cooper or aluminum slug is placed behind the semiconductor die to help carry heat away from the device. This technology seems to reach a practical limit at about ten watts per device.
In response, a number of manufacturers have begun using known chip-on-board, or COB, technology, to place a large number of LED dies in a very small area. The LEDs are mounted on an aluminum or copper core board so that the heat can be effectively transferred to a heat sink. While modules in the thirty to 300 watt range are now fairly common, several manufacturers claim modules in excess of 1000 watts are possible with current technology. High power modules, particularly modules having an input power of 30 watts, or more, create new challenges for fixture designers as significant amounts of heat must be dissipated into the environment. While theoretically traditional aluminum heat sinks could be constructed to take advantage of convection cooling for any foreseeable power level, at some point the amount of aluminum required would be prohibitive from both a cost standpoint and a fixture-handling standpoint.
Already, LED fixtures incorporating high-power LED modules have turned to forced air cooling, typically using a conventional rotating fan. With increased air flow, smaller and lighter heat sinks can be used, far offsetting the additional cost of the fan. In applications where noise is an issue, fixture designers strive for laminar air flow and thermostatically controlled fans to minimize noise. Heat pipes, water cooling, as well as other exotic techniques have been used to extract the heat from the relative small area around the LED module and dissipate it in a much larger volume.
Unfortunately, problems still exist in the use of such high power modules, such as: white LEDs exhibit a slight color shift as the operating temperature changes; and COB modules survive only a limited number of thermal cycles. Color shift over changes in operating temperature is of particular concern in motion picture and television production. While the human eye will adjust to such changes, film and video are unforgiving. The process for fixing a color shift in post-production is costly and time consuming. Adding to the problems is, the larger the heat sink, the longer it will take the fixture to achieve a steady-state temperature. In an LED fixture, it is not uncommon to see a 15 to 45 minute delay in achieving steady-state.
With regard to temperature cycling, it has been observed that, where LED fixtures are used only a few hours a day, modules will predominantly fail from temperature cycling as opposed to failure of the die or the phosphor.
Thus it is an object of the present invention to provide a system and method to reduce the effect of temperature cycling on the color temperature of the emitted light and on the life of a high power module.